The invention relates to nuclear fuel assemblies, and more particularly to fuel assemblies comprising a skeleton that constitutes the structure of the assembly and that holds a bundle of fuel elements at nodes in a regular array. Such assemblies are used, in particular, in reactors that are cooled and moderated by light water. The skeleton then comprises two end pieces interconnected by guide tubes which carry grids that are distributed along the guide tubes and that retain the fuel elements at the nodes of the array.
The guide tubes are designed to receive elements belonging to absorbent clusters for controlling the reactivity and/or to clusters for varying the energy spectrum of neutrons. To attenuate the shock caused by the clusters dropping in the event of an emergency stop of the reactor, the guide tubes are often designed to act as hydraulic dampers by providing them with bottom plugs that are formed with respective narrow passages. Such a plug may also be used to receive the guide tube to the lower end piece (FR-A-2,469,777).
In general, the end pieces are made of a material having high mechanical properties, such as stainless steel or one of the alloys known under the name Inconel, while the guide tubes are made of a zirconium base alloy. Consequently, it is difficult to secure the tubes to the end pieces directly by welding. In addition, it is desirable for the connection of the guide tubes to at least one of the end pieces to be easily disassembled and reassembled so as to allow faulty elements to be replaced.